Production of alkoxides

ABSTRACT

This invention relates to the production of alkoxides by passing an electric current through a liquid medium comprising a solution of a monohydric alcohol and as a supporting electrolyte an ammonium or quaternary ammonium salt using an anode and passing the electric current through the medium until the alkoxides are present in the liquid medium.

W111id States Patent 1 [111 3,73fifl57 Tripp May 1, 1973 [5 PRODUCTION OF ALKOXIDES [56] References Cited [75] Inventor: Terence Gordon Tripp, Chester, En- UNITED STATES PATENTS land 3,51 1,762 5/1970 Childs ..204/59 Assignee: Monsanto Chemicals Limited, Lon 3,394,059 7/1968 Young ..204/59 don, England FOREIGN PATENTS OR APPLICATIONS [22] Filed: Apr. 26,1971 1,136,016 12/1968 Great Britain ..204/59 21 A l. N I37, 13 1 pp 0 6 Primary Examiner-F. C. Edmundson Att0rney-1-lerbert B. Roberts, Roy J. Klostermann [30] Foreign Application Priority Data and Neal E. Willis May 5, 1970 Great Britain ..2 l ,644/70 [57] ABSTRACT [52] C] 59 R, 204/59 QM, 260/4482Y This invention relates to the production of alkoxides 260/429 R by passing an electric current through a liquid medium [51 1 Int CL 29/06 C07f 77/04 CO-If 7/OO comprising a solution of a monohydric alcohol and as [58] Field of Search .1 204/5912 59 QM suppming elecrolyte ammonium quaternary ammonium salt using an anode and passing the electric current through the medium until the alkoxides are present in the liquid medium.

8 Claims, N0 Drawings PRODUCTION OF ALKOXIDES BACKGROUND OF THE INVENTION filed Feb, 10, 1970, describes a process for the production of a silicon alkoxide (alkyl silicate) having from one to three carbon atoms per alkoxy group, in which an electric current is passed through a liquid medium comprising a solution, in a monohydric alcohol having from one to three carbon atoms per molecule, of an acid or a metal salt that is compatible with the said monohydric alcohol and is ionized to a sufficient extent and is present in sufficient amount to function as asupporting electrolyte, the said medium containing not more than 3 percent of water based on the weight of the monohydric alcohol, using an anode comprising silicon in contact with the liquid medium, and passage of the electric current is continued until the silicon alkoxide persists in the liquid medium and for a period thereafter.

SUMMARY OF THE INVENTION It has now been found that ammonium and quaternary ammonium salts can function as supporting electrolytes in such a process, and that when these salts are used, the process can be applied to the production of alkoxides of other elements in addition to silicon alkoxides.

DETAILED DESCRIPTION The process of the present invention is one for the production of an alkoxide of an element in Group IV or Va of the Periodic Table according to Mendeleeff, and which has an atomic number of from 14 to 82, or of a transition element in the fourth period, and in Group Ib, IIb, III a, VIa, VIIa or VIII of the Periodic Table ac- -cording 'to Mendeleeff, which comprises passing an electric current through a substantially anhydrous liquid medium comprising a solution, in a monohydric alkanol having from one to four carbon atoms per molecule, of an ammonium or a quarternary ammonium salt that is ionized to a sufficient extent and is present in sufficient amount to function as a supporting electrolyte, using an anode comprising the element in contact with the liquid medium.

The process of the invention is particularly useful for the preparation of alkoxides, for example the methoxides, ethoxides and isopropoxides, of silicon, germanium, zirconium, titanium, chromium, iron, zinc and tantalum. The alkoxides of such elements have a variety of uses. For example, silicon alkoxides (alkyl silicates) are useful as binders in the production of refractory articles such as moulds for metal casting. The alkoxides of zirconium, titanium and tantalum can be used as binders in the production of moulds for casting metals having melting points higher than silica, or for metals which react with silica at their casting temperatures. Titanium and zirconium alkoxides are useful as gelling, curing and drying agents for naturaland epoxy resins, and as components of heabresistant paints, water-repellant compositions, and anti-fouling compositions. Alkoxides of titanium, zirconium, chromium and iron are useful in the production of catalyst compositions containing these metals. Hitherto known methods of making these alkoxides have often involved several stages of synthesis, but by the present process they are produced directly from the element.

The monohydric alkanol employed in the process can be for example methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol or a mixture of these alkanols. Alkanols containing from one to three carbon atoms per molecule are preferred, and particularly good results are obtained with ethanol.

The ammonium and quarternary ammonium salts useful in the process include ammonium chloride, ammonium nitrate, quaternary ammonium salts of the formula where R R R and R are each alkyl, aryl, cycloalkyl or aralkyl, or R and R or R,, R and R together with the nitrogen atom represent a heterocyclic ring, and X is chloride, bromide, iodide or half a sulphate, such as tetramethylammonium chloride, tetramethylammonium bromide, tetraethylammonium chloride, tetraethylammonium bromide, dimethylpiperidinium bromide, methylpyridinium chloride, ethylpyridinium chloride, trimethylphenylammonium chloride, tetramethylammonium sulphate, benzyltrimethylammonium iodide. Tetramethylammonium chloride is the preferred quaternary ammonium salt. The ammonium salt or the quaternary ammonium salt must be anhydrous and must have sufficient solubility and ionization in the liquid medium to produce a solution which has sufficient conductivity to act as a current carrier. The amount of such a salt which needs to be dissolved in the liquid medium to give a practical conductivity obviously depends on a number of factors. However, a practical lower limit for the specific conductivity of the liquid is about 2.4 X 10 ohm". At lower conductivities the process is unduly prolonged. Preferably the liquid medium has a specific conductivity of at least 3.0 X 10"", for example a conductivity within the range 10' to 10 ohm cm such as for instance from 3 X 10" to 5 X 10" ohm" cm.

Satisfactory results are obtained using substantially saturated solutions of ammonium chloride in ethanol or solutions containing about 6 to 12 grams of tetramethylammonium chloride per liter of ethanol.

The anode employed may consist of the substantially pure element but the presence of small amounts of'impurity is not deleterious to the process, and alloys are sometimes preferred for economic reasons. Also, in some instances, the conductivity of the pure element is impractically low. Thus in general, the resistivity of the anode material should not exceed 10 ohm-cm.

In the case of silicon, a compound or alloy of silicon rather than a semiconductor grade of the element is preferably used. Ferrosilicons have been found to be especially suitable. The silicon content of the ferrosilicon can, for example, be within the range 60 to 99 percent or to 77 percent by weight, and in typical instances may be within the ranges 70 80 percent, 90 95 percent or 97 99 percent by weight. In the case of titanium, zirconium, germanium and tantalum pieces of scrap metal or compressed scrap metal, with or without undergoing a preliminary cleaning procedure, can be used.

Any inert conductive material can serve as the cathode. Graphite is a useful cathodic material having a considerable advantage in terms of cost over other functionally suitable materials such as platinum or silver.

The electric current used in the process of the invention can be a simple direct current or rectified A.C. with or without smoothing. The current density at the anode may typically be within the range of from 7 to 200 milliamps per square centimeter, although operation at current densities over a wider range than this is possible. For economic reasons, the voltage applied in excess of the decomposition voltage of the system should be kept as low as possible. In practice, the operating voltage is determined by factors such as cell design and electrolyte concentration, and wide variations are possible.

The temperature at which the electrolysis is conducted is not critical. The generally most convenient manner of operating is to provide the electrolysis cell with a reflux condenser and to carry out the process at the boiling point of the liquid medium. Often the resistive heating by the current is sufficient to maintain the liquid medium at its boiling point. The process can be conducted at lower temperatures, however, for example from 10 C. up to the boiling point of the liquid medium, but it is then generally necessary to provide cooling means.

It is usual to carry out the electrolysis in the presence of an inert atmosphere, e.g., nitrogen, but this is not essential. The liquid medium and the current carrier must be anhydrous since any water present would hydrolyze the alkoxide to an oxide sol. An alkoxide could be prepared by passing a current through a liquid medium containing not more than 3 percent of watenby continuing the electrolysis until all the water is consumed and then continuing the electrolysis to form the alkoxide. However, such a process is obviously inefficient and the pure product is more difficult to isolate from the crude reaction product. The presence of water would also cause passivity of the element in some cases.

If desired the liquid medium may contain an inert organic liquid miscible with the liquid medium, e.g., a ketone, an ether or an ester as a liquid diluent.

For most applications, preferably at least 80 percent and more preferably at least 90 percent of the total weight of the liquid medium is an alcohol having from one to three carbon atoms. For some applications, however, the liquid medium can contain inert solvents up to percent based on the weight of the liquid medium, i.e., acetone. Such solvents include ketones, higher alcohols ethers and esters, for example 2-ethylhexanol, hexanol or pentanol, glycols such as ethylene glycol or propylene glycol, or glycol ethers, such as ethylene glycol monoethyl ether or diethylene glycol monoethyl ether. Alkoxides containing these solvents are preferably prepared by gradual addition of the solvent to the medium from which the alcohol containing one to three carbon atoms per molecule is being evaporated.

product is diluted with an inert anhydrous organic solvent such as diethyl ether, the precipitated current carrier is filtered off, the solvent and ethanol are evaporated and the residue is either purified by distillation under reduced pressure or used without further purification. Alkoxides such as zirconium ethoxide which are solid at room temperature can be purified by recrystallization from anhydrous solvents or by distillation.

The invention is illustrated by the following examples. The ethanol used in the examples was dried over molecular sieves. The yields quoted in the following examples are calculated from the weight of oxide obtained when a sample of known volume is taken from the solution obtained after the electrolysis and hydrolyzed and the precipitate is filtered off and ignited.

EXAMPLE 1 The preparation of germanium ethoxide.

A hot saturated solution of ammonium chloride in anhydrous ethanol was electrolyzed under an atmosphere of nitrogen in a flask fitted with a graphite cathode, a germanium anode and a reflux condenser. A direct current of 2 amps was passed through the cell which maintained the electrolyte at its boiling point by resistive heating. Towards the end of the run the current had fallen to 0.7 amps. The applied voltage was 40-60 volts and a total of 32 amp. hours was supplied to the cell during the run. The resultant solution was found to contain 61 g. of germanium ethoxide which represents a current efficiency of 78 percent. Evaporation of the ethanol from an aliquot of this solution and distillation of the residue gave germanium ethoxide b.p. 80 C./l3 mm. Hg. The alkoxide was found to have a GeO content of 41 percent by weight by hydrolyzing a sample and pyrolyzing the product compared with the theoretical GeO content of 41.5 percent. The n.m.r. spectrum of the alkoxide was consistent with the formula Ge(OEt) Quantitative n.m.r. using benzene as an internal standard, gave a hydrogen content of 7.4 percent compared with the theoretical content of 7.9 percent.

EXAMPLE 2 The preparation of tantalum ethoxide.

A hot saturated solution of ammonium chloride in anhydrous ethanol was electrolyzed under an atmosphere of nitrogen in a flask fitted with a graphite cathode, a tantalum anode and a reflux condenser. A direct current of between 0.4 and 0.25 amps was maintained throughout the run after reversing the polarity of the electrodes for 20 minutes. The applied voltage was 50 volts and a total of 6 amp. hours was supplied to the cell during the run. The resultant solution was found to contain 17.6 g. of tantalum ethoxide which represents a current efficiency of 97 percent. Evaporation of the ethanol from an aliquot of this solution and distillation of the residue at 1 mm. Hg. gave tantalum ethoxide. The alkoxide had a Ta,O content of 54 percent by weight compared with the theoretical content of 55 EXAMPLE 3 The preparation of titanium ethoxide.

A hot saturated solution of ammonium chloride in anhydrous ethanol was electrolyzed under an atmosphere of nitrogen in a flask fitted with a graphite cathode, a titanium anode and a reflux condenser. A direct current of 2 amps at 55-65 volts was passed which fell after 2 hours to 0.15 amps at 80 volts. The electrolyte contained titanium ethoxide and gave a soft gel when mixed with water.

EXAMPLE 4 The preparation of zirconium ethoxide.

A hot saturated solution of ammonium chloride in anhydrous ethanol was electrolyzed under an atmosphere of nitrogen in a flask fitted with a graphite cathode, a zirconium anode and a reflux condenser. A direct current of 0.5 amps was passed which slowly fell to 0.3 amps. The electrolysis was continued until the ZrO content of the electrolyte rose to about 10 percent by weight. The resultant solution was found to contain 12.9 g. of zirconium ethoxide which represents a current efficiency of 82 percent. Evaporation of the ethanol from an aliquot of the solution and distillation of the residue gave zirconium ethoxide b.p. 150-200 C./1 mm. Hg. as a waxy solid which had a ZrO content of 45.3 percent compared with the theoretic value 45.4 percent.

EXAMPLE 5 The ethanol was evaporated from an aliquot of the solution and distillation under reduced pressure gave silicon ethoxide b.p. 60-80/25 mm. of Hg. having a SiO content of 25 percent by weight compared with the theoretical value for Si(OEt) of 28.8 percent.

EXAMPLE 6 The preparation of titanium ethoxide.

A hot solution of anhydrous tetramethylammonium chloride (lg) in anhydrous ethanol (100 ml) was electrolyzed under an atmosphere of nitrogen in a flask fitted with a graphite cathode and a titanium anode. A direct current of 0.75 amps was passed for 10 amp hours to give a solution containing 18 g. of titanium ethoxide which represents a current efficiency of 91 percent. The solution was filtered, the ethanol was evaporated from the filtrate, and the residue was distilled under reduced pressure to give titanium ethoxide b.p. 136-l42 C./ 2-3 mm. Hg. as a solid. This material had a TiO content of 33.3 percent compared with the theoretical value of 35.1 percent. The current efficiency of the process was 91 percent based on the TiO content of the crude reaction product at theend of the run and was 109 percent based on the loss in weight of the anode.

EXAMPLE 7 The preparation of silicon ethoxide.

A hot solution of anhydrous tetramethylammonium chloride (1g) in anhydrous ethanol (100 ml) was electrolyzed under an atmosphere of nitrogen in a flask fitted with a graphite cathode and a ferrosilicon anode. A direct current of 1.2 amps at 72 volts was passed and the current gradually fell. The run was stopped after 15 amp hours when the current had fallen to 0.8 amps. The resultant solution was filtered, the ethanol was evaporated from the filtrate, and the residue was distilled under reduced pressure to give ethyl orthosilicate containing some ethyl polysilicate.

What we claim is:

1. A process for the production of an alkoxide of an element in Group IV or Va of the Periodic Table according to Mendeleeff, and which has an atomic number of from 14 to 82, or of a transition element in the fourth period and in Group lb, llb, Illa, Vla, Vlla or VIII of the Periodic Table according to Mendeleeff, which comprises passing an electric current through a substantially anhydrous liquid medium comprising a solution, in a monohydric alkanol having from one to four carbon atoms per molecule, of a salt selected from the group consisting of ammonium chloride, ammonium nitrate and a quaternary ammonium salt of the forwhere R R R and R are each alkyl, aryl, cycloalkyl or aralkyl, or R and R or R,, R and R together with the nitrogen atom represent a heterocyclic ring, and X is chloride, bromide, or iodide that is ionized to a sufficient extent and is present in sufficient amount to function as a supporting electrolyte, in which the anode comprising the element from which the alkoxide is formed.

2. A process according to Claim 1, in which the liquid medium is a solution having a specific conductivity of at least 3.0 X 10" ohm cm at the operating temperature.

3. A process according to Claim 2, in which the liquid medium has a specific conductivity within the range 10 to 10' ohm cm".

4. A process according to claim 1, in which the salt is ammonium chloride.

5. A process according to claim 1, in which the element is selected from the group .consisting of silicon, germanium, zirconium, titanium and tantalum.

6. A process according to claim 5, in which the element is silicon and the anode is a ferrosilicon containing from 60 to 99 percent by weight of silicon.

7. A process according to claim 6, in which the monohydric alkanol is ethanol.

'8. A process according to claim 1 in which the quaternary ammonium salt is tetramethylammonium chloride. 

2. A process according to Claim 1, in which the liquid medium is a solution having a specific conductivity of at least 3.0 X 10 5 ohm 1 cm 1 at the operating temperature.
 3. A process according to Claim 2, in which the liquid medium has a specific conductivity within the range 10 4 to 10 3 ohm 1 cm
 1. 4. A process according to claim 1, in which the salt is ammonium chloride.
 5. A process according to claim 1, in which the element is selected from the group consisting of silicon, germanium, zirconium, titanium and tantalum.
 6. A process according to claim 5, in which the element is silicon and the anode is a ferrosilicon containing from 60 to 99 percent by weight of silicon.
 7. A process according to claim 6, in which the monohydric alkanol is ethanol.
 8. A process according to claim 1 in which the quaternary ammonium salt is tetramethylammonium chloride. 